Surface modifications, especially polishing, are a required process in many industrial production areas. These include:                Automotive, e.g. polishing of moulds for plastic parts such as boot lids and light covers and lenses.        Engine parts such as cam axes.        Medical implants, e.g. the surface of artificial hip joints.        Optics such as injection moulds for safety glasses and contact lenses.        
Such polished surfaces can be several square meters in size and require a surface accuracy of 1 micrometer or less. Today, the sample is moved from the polishing process to a laboratory where the surface is measured at small areas at a time in a time-consuming process. Furthermore, most of the polishing is done manually, where the manual polisher establishes the quality by looking at the tool/mould, whereas the automatic procedure using robot or laser has, in general, to be checked by taking the tool/mould out of the process chamber.
In order to check the surface accuracy, several methods are known in the art.
U.S. Pat. No. 6,084,671 discloses a method and apparatus for surface analysis using Gaussian beam profiles. A Gaussian beam is shaped with a lens system and illuminates a surface. The reflected light is distorted by surface irregularities. The reflected beam is then observed through a series of spatial filters which basically remove the original Gaussian beam shape. The measured beam with difference in residual beam shape is the recorded with a camera. This solution requires that the surface is illuminated by a very specific beam pattern in the form a Gaussian shape. Further, the illumination of the surface is uneven over the surface in that the edges of the studied area is less intensely illuminated than the center and any statistical measure of the surface structure is therefore biased towards the center.
US 2005/0046870 disclose a method for characterizing surfaces by illuminating it at an angle to show shadowgraph of larger structure on the surface. This solution describes and discusses geometrical reflections only. It does not relate to illuminating polished surfaces with monochromatic light. Further, the illumination is not directed perpendicularly to the surface.
US 2012/107971 discloses a polishing pad assembly for a chemical mechanical polishing apparatus. The polishing pad has a polishing surface and a surface opposite the polishing surface for attachment to a platen, and a solid light-transmissive window formed in the polishing pad. The light-transmissive window is more transmissive to light than the polishing pad. The light-transmissive window has a light-diffusing bottom surface. This solution requires a chemical transparent top layer over the surface to produce interference with the surface reflected wave. This is a single spot measurement instrument and technique which measures interference between two overlapping surfaces.
U.S. Pat. No. 4,873,434 discloses a scanning optical microscope which comprises a source of optical radiation and means for focusing radiation from the source into an interrogating spot on a surface under examination. The spot is deflected about a point on the surface, and the surface topography is measured by measuring the amplitude and/or phase of the radiation reflected from the surface at the spot position. This technique measures one spot at a time. Each spot is handled separately and does not give any direct quantitative analysis of a larger area.
U.S. Pat. No. 5,917,191 discloses a method for measuring surface topography characterized by making multiple scans of the surface with a laser scanning unit and utilizing the multiple scans to create representations of the surface's topography. The surface topography data can also be used to calculate the compressive or tensile stress caused by a thin film applied to the surface of a semiconductor wafer. The apparatus of the solution scans a laser beam across a surface in an x direction, and detects displacements of a reflected portion of the laser beam in a z direction. A pair of photo detectors is used to translate z direction displacements of the reflected beam into analog signals which are digitized and input into a microcomputer for analysis. The multiple scans of the surface are preferably accomplished by placing the work piece on a pedestal which can be rotated to various angular positions. This technique is scanning point by point to give point topographic data. It neither describes nor teaches quantitative analysis over a large area simultaneously.